Ace the Certified Ophthalmic Assistant Exam 2025 – Eye-Opening Success Awaits!

The iris plays a crucial role in regulating the amount of light that enters the eye. It is the colored part of the eye and is made up of muscle fibers that can constrict or dilate the pupil, the opening in the center of the iris. When exposed to bright light conditions, the iris contracts, causing the pupil to become smaller. This contraction serves as a protective mechanism to reduce the amount of light reaching the retina, thus preventing potential damage from excessive brightness and enhancing visual acuity.

In contrast, the canthi refer to the corners of the eye where the upper and lower eyelids meet, which do not have a role in light regulation. The ciliary body is primarily responsible for controlling the shape of the lens for focusing and does not participate directly in light regulation. The medial rectus is one of the extraocular muscles that helps control eye movement, but it does not have any function in contraction due to lighting conditions. Therefore, the iris is the only part of the eye that contracts in bright light, making it the correct answer.